This invention relates to medical electrical stimulation electrodes in general and to epicardial defibrillation electrodes in particular.
In the past years, there has been substantial activity directed toward development of a practical, implantable defibrillator. Some approaches, such as this disclosed in U.S. Pat. No. 3,942,536, issued to Mirowski et al, and U.S. Pat. No. 4,161,952, issued to Kinney et al have employed only endocardial electrodes. However, more recent approaches to this problem have focused on systems employing one or more epicardial electrodes as alternatives to or in addition to endocardial electrodes. Some such systems are disclosed in U.S. Pat. No. 4,030,509, issued to Heilman et al, and U.S. Pat. No. 4,291,707, issued to Heilman et al.
Generally, an epicardial defibrillation electrode must accomplish two important functions. First, it must deliver a relatively large amount of electrical energy to the heart with a minimal amount of tissue damage. For this reason, it is generally believed that epicardial defibrillation electrodes should have a large electrode surface area in order to distribute the energy over a wide area of the heart. The use of large surface electrodes reduces the density of the current applied to the heart, reducing the potential for damage to heart tissue. In addition, use of electrodes extending over a large area of the heart is believed to assist in improving current distribution through the heart tissue, reducing resistance and reducing the amount of energy applied to the heart. For this reason, many prior art designs have employed electrodes having large surface areas and having individual conductive areas dispersed over the surface of the electrode.
The large size of epicardial defibrillation electrodes itself creates a problem. Motion of the heart during contraction is complex, and has been likened to a "wringing" action. A large surface area electrode must be capable of conforming to the contours of the heart and to changes in contours of the heart in order to continue to function properly. Typical prior art defibrillation patch electrodes, as disclosed in U.S. Pat. No. 4,030,509, issued to Heilman et al, however, employ large surface electrodes in the form of screens or plates which limit the flexibility of the electrode patch. One alternative to the use of screens and plates is disclosed in U.S. Pat. No. 4,641,656 issued to Smits et al which discloses electrode pads having spaced contact areas separated by perforations or indentations which allow the individual conductive areas to move with respect to one another.